The present invention generally relates to antiskid control devices for preventing the skid of wheels of a motor vehicle and more particularly, to/an antiskid control device for preventing occurrence of a phenomenon that, when estimated brake fluid pressures of wheel cylinders increase extremely according to a driver's force for stepping on a brake pedal, proper antiskid control is not performed, thereby resulting in instability of a vehicle body.
Generally, in prior art antiskid control devices, the skid state of wheels is detected from the amount of drop of wheel speeds relative to the vehicle body speed, and wheel acceleration or deceleration and brake fluid pressure is controlled to a proper level in accordance with the skid state. Thus, since the skid of the wheels is maintained at a proper level, i.e., a region where the coefficient of friction of a road surface comes close to its peak, braking distance is reduced and excellent driving stability of a vehicle body is secured.
However, if only behaviors of the wheels and the vehicle body are considered for control of brake fluid pressure through its increase or reduction, a problem may arise in that brake fluid pressure becomes insufficient due to excessive reduction of brake fluid pressure or due to improper increase of brake fluid pressure after excessive reduction of brake fluid pressure.
Meanwhile, in order to control brake fluid pressure, on-off type solenoid controlled valves have been generally employed to increase and decrease brake fluid pressures in wheel cylinders. In this case, since increase and reduction characteristics of brake fluid pressure change according to brake fluid pressure in a master cylinder or each wheel cylinder, the above mentioned problem may be encountered if accurate brake fluid pressure is not grasped.
In order to solve this problem, Japanese Patent Laid-Open Publication Nos. 2-171377 (1990) and 3-92463 (1991) disclose that an actuator having a servo function is used for directly controlling brake fluid pressure. However, since this actuator is expensive, the actuator is not desirable in view of its cost.
Thus, Japanese Patent Laid-Open Publication No. 5-246317 (1993) discloses a technique in which brake fluid pressure in each wheel cylinder during antiskid control is estimated by using an actuator having a general on-off type solenoid controlled valve so as to control brake fluid pressure. However, in this known technique, estimated brake fluid pressure and the brake fluid pressure in each wheel cylinder changes as shown in FIG. 1 in a case where a driver steps on a brake pedal strongly so as to perform antiskid control. On the other hand, when the driver steps on the brake pedal so lightly as to possibly cause antiskid control on a road surface on which his motor vehicle is running, brake fluid pressure in the master cylinder is low as shown in FIG. 2. Therefore, actual brake fluid pressure in each wheel cylinder does not rise so high as to initiate a brake fluid pressure increasing command given to the solenoid controlled valve, so that estimated brake fluid pressure becomes excessively large and a pressure reducing command leads to a small brake fluid pressure reducing value suitable for a road surface having a higher coefficient of friction, thereby resulting in instability of the vehicle body.